Dispersion strengthening has been recognized in the past as a method for increasing strength and hardness of metals. A solid solution alloy comprising relatively noble matrix metal having relatively low heat or free energy of oxide formation and a solute metal having relatively high negative heat or free energy of oxide formation wherein the alloy is heated under oxidizing conditions to preferentially oxidize the solute metal. This technique is known in the art as in situ internal oxidation of the solute metal to the solute metal oxide or more simply "internal oxidation."
Dispersion-strengthened metal products, such as copper dispersion strengthened with aluminum oxide, have many commercial and industrial uses wherein high temperature strength properties and high electrical and/or thermal conductivities are desired or required in the finished product. Such commercial uses include frictional brake parts such as linings, facings, drums, and other machine parts for friction applications. Other commercial uses include electrical contact points, resistance welding electrodes, electrodes generally, electrical switches and switch gears, transistor assemblies, wires for solderless connections, wires for electrical motors, and many other uses requiring good electrical and thermal conductivities together with good strength and hardness at elevated temperatures.
Several prior art processes for internal oxidation have been suggested, such as disclosed in the Schreiner patent, U.S. No. 3,488,185; the McDonald patent, U.S. No. 3,552,954; and the Grant patent, U.S. No. 3,179,515. The prior art processes require delicate control over the partial pressure of oxygen during oxidation, or require removal of an oxidant residue which otherwise would form defects in the dispersion-strengthened metal.
Copending application now being U.S. Pat. No. 3,779,714 provides an improved alloy-oxidant mixture by providing for assimilation of the oxidant residue into the dispersion-strengthened metal wherein the oxidant residue is dispersion strengthened during thermal coalescence by a hard, refractory metal oxide provided in the oxidant. The oxidant residue formed during internal oxidation is not required to be removed from the dispersion-strengthened metal but rather is dispersion strengthened by the hard, refractory metal oxide during coalescence to form an integral part of the dispersion-strengthened metal stock.
It has been found that dispersion-strengthened metals produced by internal oxidation have substantially improved properties if the alloy powder is recrystallized prior to internal oxidation in order to increase the grain size of the alloy powder to at least ASTM Grain Size No. 6 as measured by ASTM Test No. E-112.
Accordingly, it is a primary objective of this invention to increase the grain size and reduce the grain boundary area in the alloy powder prior to the step of internal oxidation in processes for dispersion strengthening of metals.
A further objective and advantage of this invention is to provide reduced grain boundaries to substantially eliminate the concentration of solute metal oxide at the grain boundaries of the alloy during internal oxidation. This provides a dispersion-strengthened metal product having sustained resistance to preferential stress failure at the grain boundaries.
A still further objective is to provide a dispersion-strengthened metal product having a finer and more uniform distribution of dispersoid particles which result in improved elevated temperature properties.
A further object of this invention is to minimize formation of inner oxide film of solute metal oxide within the alloy during internal oxidation by increasing the grain size of the alloy prior to internal oxidation which advantageously permits higher oxidation temperatures as well as decreasing oxidation time.
A still further objective is to permit increased amounts of solute metal oxide which advantageously provide improved strength properties particularly at elevated temperatures.
A still further objective is to dispersion strengthen larger alloy particles which advantageously permits increased yield.
These and other advantages will become more apparent from the detailed description of the invention.